This invention relates to thermal inkjet printers, and more particularly to printmodes.
Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R.C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes xe2x80x9cdot locationsxe2x80x9d, xe2x80x9cdot positionsxe2x80x9d, or pixelsxe2x80x9d. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle, and has a firing resistor located in the chamber. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
In an inkjet printhead the ink is fed from an ink reservoir integral to the printhead or an xe2x80x9coff-axisxe2x80x9d ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various vaporization chambers either through an elongated hole formed in the center of the bottom of the substrate, xe2x80x9ccenter feedxe2x80x9d, or around the outer edges of the substrate, xe2x80x9cedge feed.xe2x80x9d
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the resistors is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as high as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the height of the swath or a fraction thereof, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Lines, text and graphics are normally printed with uniform density. In one or two pass printmodes, this results in a high firing frequency for black and saturated colors. High firing frequency has a negative effect on the drops that are ejected: drop velocity, drop volume, drop shape and drop trajectory. Output printed with high frequency and uniform density text and lines exhibits defects that are the result of the sub-optimal firing conditions. Inkjet printheads often have frequency dependant drop defects, such as spray, spear drops and tails. The effects of these drop defects on image quality can vary with scan direction due to aerodynamics, burst length (number of drops fired in a row at high frequency) and other factors. A previous approach to this problem uses image processing to improve edge quality by reducing the firing frequency at the edges of lines and text characters. See, U.S. patent application Ser. No. 09/562,264, filed April 29, 2000, entitled xe2x80x9cPrint Mode for Improved Leading and Trailing Edges and Text Print Quality.xe2x80x9d This method is effective, but requires image processing which can be expensive or time consuming.
Accordingly, there is a need for a new solution to the problem of text and graphics degradation and, more generally, edge roughness that is associated with high frequency firing.
This invention increases text, line and graphics quality by reducing edge roughness caused by defects such as tails, spray or spear drops. A method for incremental printing with an inkjet swath printer by ejecting drops of an ink from a printhead while scanning the printhead over a print medium, including determining a first edge and a second edge of each pixel in each row with respect to a first scanning direction. Moving the printhead in the first scanning direction while ejecting the drops of ink on the first edge of the pixels at a first firing frequency and on the second edge of the pixels at a second firing frequency and moving the printhead in a second direction opposite to the first scanning direction while ejecting the drops of ink on the second edge of the pixels at the first firing frequency and on the first edge of the pixels at the second firing frequency.